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<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
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<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
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<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
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</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="functions.html" title="cpp/language/functions"> Functions</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html" title="cpp/language/function"> function declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html#Parameter_list" title="cpp/language/function"> function parameter list</a> </td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="lambda.html" title="cpp/language/lambda"> lambda expression</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="coroutines.html" title="cpp/language/coroutines"> coroutines </a> <span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Function calls </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> argument-dependent lookup (ADL)</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_other.html#Built-in_function_call_operator" title="cpp/language/operator other"> function-call operator</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../named_req/FunctionObject.html" title="cpp/named req/FunctionObject"> function objects</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Overloading </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="overload_resolution.html" title="cpp/language/overload resolution"> overload resolution</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operators.html" title="cpp/language/operators"> operator overloading</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="overloaded_address.html" title="cpp/language/overloaded address"> address of an overload set</a> </td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Argument-dependent lookup, also known as ADL, or Koenig lookup <sup id="cite_ref-1" class="reference"><a href="adl.html#cite_note-1">[1]</a></sup>, is the set of rules for looking up the unqualified function names in <a href="operator_other.html" title="cpp/language/operator other">function-call expressions</a>, including implicit function calls to <a href="operators.html" title="cpp/language/operators">overloaded operators</a>. These function names are looked up in the namespaces of their arguments in addition to the scopes and namespaces considered by the usual <a href="lookup.html" title="cpp/language/lookup">unqualified name lookup</a>.
</p>
<div class="t-example"><p>Argument-dependent lookup makes it possible to use operators defined in a different namespace. Example:</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Test<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// There is no operator&lt;&lt; in global namespace, but ADL</span>
                           <span class="co1">// examines std namespace because the left argument is in</span>
                           <span class="co1">// std and finds std::operator&lt;&lt;(std::ostream&amp;, const char*)</span>
    operator<span class="sy1">&lt;&lt;</span><span class="br0">(</span><a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a>, <span class="st0">"Test<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same, using function call notation</span>
 
    <span class="co1">// however,</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> endl<span class="sy4">;</span> <span class="co1">// Error: 'endl' is not declared in this namespace.</span>
                       <span class="co1">// This is not a function call to endl(), so ADL does not apply</span>
 
    endl<span class="br0">(</span><a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: this is a function call: ADL examines std namespace</span>
                     <span class="co1">// because the argument of endl is in std, and finds std::endl</span>
 
    <span class="br0">(</span>endl<span class="br0">)</span><span class="br0">(</span><a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: 'endl' is not declared in this namespace.</span>
                       <span class="co1">// The sub-expression (endl) is not a function call expression</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id="Details">Details</span></h3>
<p>First, the argument-dependent lookup is not considered if the lookup set produced by usual <a href="lookup.html" title="cpp/language/lookup">unqualified lookup</a> contains any of the following:
</p>
<div class="t-li1"><span class="t-li">1)</span> a declaration of a class member</div>
<div class="t-li1"><span class="t-li">2)</span> a declaration of a function at block scope (that's not a <a href="namespace.html#Using-declarations" title="cpp/language/namespace">using-declaration</a>)</div>
<div class="t-li1"><span class="t-li">3)</span> any declaration that is not a function or a function template (e.g. a function object or another variable whose name conflicts with the name of the function that's being looked up)</div>
<p>Otherwise, for every argument in a function call expression its type is examined to determine the <i>associated set of namespaces and classes</i> that it will add to the lookup.
</p>
<div class="t-li1"><span class="t-li">1)</span> For arguments of fundamental type, the associated set of namespaces and classes is empty</div>
<div class="t-li1"><span class="t-li">2)</span> For arguments of class type (including union), the set consists of</div>
<div class="t-li2"><span class="t-li">a)</span> The class itself</div>
<div class="t-li2"><span class="t-li">b)</span> All of its direct and indirect base classes</div>
<div class="t-li2"><span class="t-li">c)</span> If the class is a <a href="nested_classes.html" title="cpp/language/nested types">member of another class</a>, the class of which it is a member</div>
<div class="t-li2"><span class="t-li">d)</span> The innermost enclosing namespaces of the classes added to the set</div>
<div class="t-li1"><span class="t-li">3)</span> For arguments whose type is a <a href="class_template.html" title="cpp/language/class template">class template</a> specialization, in addition to the class rules, the following types are examined and their associated classes and namespaces are added to the set</div>
<div class="t-li2"><span class="t-li">a)</span> The types of all template arguments provided for type template parameters (skipping non-type template parameters and skipping template template parameters)</div>
<div class="t-li2"><span class="t-li">b)</span> The namespaces in which any template template arguments are members</div>
<div class="t-li2"><span class="t-li">c)</span> The classes in which any template template arguments are members (if they happen to be class member templates)</div>
<div class="t-li1"><span class="t-li">4)</span> For arguments of enumeration type, the innermost enclosing namespace of the declaration of the enumeration type is defined is added to the set. If the enumeration type is a member of a class, that class is added to the set.</div>
<div class="t-li1"><span class="t-li">5)</span> For arguments of type pointer to T or pointer to an array of T, the type T is examined and its associated set of classes and namespaces is added to the set.</div>
<div class="t-li1"><span class="t-li">6)</span> For arguments of function type, the function parameter types and the function return type are examined and their associated set of classes and namespaces are added to the set.</div>
<div class="t-li1"><span class="t-li">7)</span> For arguments of type pointer to member function F of class X, the function parameter types, the function return type, and the class X are examined and their associated set of classes and namespaces are added to the set.</div>
<div class="t-li1"><span class="t-li">8)</span> For arguments of type pointer to data member T of class X, the member type and the type X are both examined and their associated set of classes and namespaces are added to the set.</div>
<div class="t-li1"><span class="t-li">9)</span> If the argument is the name or the <a href="overloaded_address.html" title="cpp/language/overloaded address">address-of expression for a set of overloaded functions</a> (or function templates), every function in the overload set is examined and its associated set of classes and namespaces is added to the set.
<dl><dd><ul><li> Additionally, if the set of overloads is named by a <a href="templates.html#template-id" title="cpp/language/templates">template-id</a>, all of its type template arguments and template template arguments (but not non-type template arguments) are examined and their associated set of classes and namespaces are added to the set.</li></ul></dd></dl></div>


 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>If any namespace in the associated set of classes and namespaces is an <a href="namespace.html" title="cpp/language/namespace">inline namespace</a>, its enclosing namespace is also added to the set.
</p><p>If any namespace in the associated set of classes and namespaces directly contains an inline namespace, that inline namespace is added to the set.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>After the associated set of classes and namespaces is determined, all declarations found in classes of this set are discarded for the purpose of further ADL processing, except namespace-scoped friend functions and function templates, as stated in point 2 below . 
</p><p>The set of declarations found by ordinary <a href="lookup.html" title="cpp/language/lookup">unqualified lookup</a> and the set of declarations found in all elements of the associated set produced by ADL, are merged, with the following special rules
</p>
<div class="t-li1"><span class="t-li">1)</span> <a href="namespace.html#Using-directives" title="cpp/language/namespace">using-directives</a> in the associated namespaces are ignored</div>
<div class="t-li1"><span class="t-li">2)</span> namespace-scoped friend functions (and function templates) that are declared in an associated class are visible through ADL even if they are not visible through ordinary lookup</div>
<div class="t-li1"><span class="t-li">3)</span> all names except for the functions and function templates are ignored (no collision with variables)</div>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>Because of argument-dependent lookup, non-member functions and non-member operators defined in the same namespace as a class are considered part of the public interface of that class (if they are found through ADL) <sup id="cite_ref-2" class="reference"><a href="adl.html#cite_note-2">[2]</a></sup>.
</p>
ADL is the reason behind the established idiom for swapping two objects in generic code:<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">using</span> <a href="../algorithm/swap.html"><span class="kw1603">std::<span class="me2">swap</span></span></a><span class="sy4">;</span>
swap<span class="br0">(</span>obj1, obj2<span class="br0">)</span><span class="sy4">;</span></pre></div></div>because calling <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../algorithm/swap.html"><span class="kw1603">std::<span class="me2">swap</span></span></a><span class="br0">(</span>obj1, obj2<span class="br0">)</span></span></span> directly would not consider the user-defined swap() functions that could be defined in the same namespace as the types of obj1 or obj2, and just calling the unqualified <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span>obj1, obj2<span class="br0">)</span></span></span> would call nothing if no user-defined overload was provided. In particular, <span class="t-lc"><a href="../algorithm/iter_swap.html" title="cpp/algorithm/iter swap">std::iter_swap</a></span> and all other standard library algorithms use this approach when dealing with <a href="../named_req/Swappable.html" title="cpp/named req/Swappable"><span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">Swappable</span></a> types.
<p>Name lookup rules make it impractical to declare operators in global or user-defined namespace that operate on types from the std namespace, e.g. a custom <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">&gt;&gt;</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span> for <span class="t-lc"><a href="../container/vector.html" title="cpp/container/vector">std::vector</a></span> or for <span class="t-lc"><a href="../utility/pair.html" title="cpp/utility/pair">std::pair</a></span> (unless the element types of the vector/pair are user-defined types, which would add their namespace to ADL). Such operators would not be looked up from template instantiations, such as the standard library algorithms. See <a href="dependent_name.html" title="cpp/language/dependent name">dependent names</a> for further details.
</p><p>ADL can find a <a href="friend.html" title="cpp/language/friend">friend function</a> (typically, an overloaded operator) that is defined entirely within a class or class template, even if it was never declared at namespace level.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> number
<span class="br0">{</span>
    number<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">friend</span> number gcd<span class="br0">(</span>number x, number y<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// definition within</span>
                                                         <span class="co1">// a class template</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// unless a matching declaration is provided gcd is</span>
<span class="co1">// an invisible (except through ADL) member of this namespace</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    number<span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span> a<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span>, b<span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
    a <span class="sy1">=</span> gcd<span class="br0">(</span>a, b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// finds gcd because number&lt;double&gt; is an associated class,</span>
                   <span class="co1">// making gcd visible in its namespace (global scope)</span>
<span class="co1">//  b = gcd(3, 4); // Error; gcd is not visible</span>
<span class="br0">}</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx20"><td>
<p>Although a function call can be resolved through ADL even if ordinary lookup finds nothing, a function call to a <a href="function_template.html" title="cpp/language/function template">function template</a> with explicitly-specified template arguments requires that there is a declaration of the template found by ordinary lookup (otherwise, it is a syntax error to encounter an unknown name followed by a less-than character)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N1
<span class="br0">{</span>
    <span class="kw1">struct</span> S <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> X<span class="sy1">&gt;</span>
    <span class="kw4">void</span> f<span class="br0">(</span>S<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw1">namespace</span> N2
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
    <span class="kw4">void</span> f<span class="br0">(</span>T t<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">void</span> g<span class="br0">(</span>N1<span class="sy4">::</span><span class="me2">S</span> s<span class="br0">)</span>
<span class="br0">{</span>
    f<span class="sy1">&lt;</span><span class="nu0">3</span><span class="sy1">&gt;</span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span>     <span class="co1">// Syntax error until C++20 (unqualified lookup finds no f)</span>
    N1<span class="sy4">::</span><span class="me2">f</span><span class="sy1">&lt;</span><span class="nu0">3</span><span class="sy1">&gt;</span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, qualified lookup finds the template 'f'</span>
    N2<span class="sy4">::</span><span class="me2">f</span><span class="sy1">&lt;</span><span class="nu0">3</span><span class="sy1">&gt;</span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: N2::f does not take a non-type parameter</span>
                 <span class="co1">//        N1::f is not looked up because ADL only works</span>
                 <span class="co1">//              with unqualified names</span>
 
    <span class="kw1">using</span> N2<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span>
    f<span class="sy1">&lt;</span><span class="nu0">3</span><span class="sy1">&gt;</span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: Unqualified lookup now finds N2::f</span>
             <span class="co1">//     then ADL kicks in because this name is unqualified</span>
             <span class="co1">//     and finds N1::f</span>
<span class="br0">}</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-until-cxx20">(until C++20)</span></td></tr>
</table>
<p>In the following contexts ADL-only lookup (that is, lookup in associated namespaces only) takes place:
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> the lookup of non-member functions <code>begin</code> and <code>end</code> performed by the <a href="range-for.html" title="cpp/language/range-for">range-for</a> loop if member lookup fails
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> the <a href="dependent_name.html#Lookup_rules" title="cpp/language/dependent name">dependent name lookup</a> from the point of template instantiation.
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<ul><li> the lookup of non-member function <code>get</code> performed by  <a href="structured_binding.html" title="cpp/language/structured binding">structured binding declaration</a> for tuple-like types
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Examples">Examples</span></h3>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br>Reason: more examples </td></tr></table>
<div class="t-example"><p>Example from <a rel="nofollow" class="external free" href="http://www.gotw.ca/gotw/030.htm">http://www.gotw.ca/gotw/030.htm</a></p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> A
<span class="br0">{</span>
    <span class="kw1">struct</span> X<span class="sy4">;</span>
    <span class="kw1">struct</span> Y<span class="sy4">;</span>
 
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">void</span> g<span class="br0">(</span>X<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw1">namespace</span> B
<span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span>
    <span class="br0">{</span>
        f<span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls B::f (endless recursion)</span>
    <span class="br0">}</span>
 
    <span class="kw4">void</span> g<span class="br0">(</span>A<span class="sy4">::</span><span class="me2">X</span> x<span class="br0">)</span>
    <span class="br0">{</span>
        g<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: ambiguous between B::g (ordinary lookup)</span>
              <span class="co1">//        and A::g (argument-dependent lookup)</span>
    <span class="br0">}</span>
 
    <span class="kw4">void</span> h<span class="br0">(</span>A<span class="sy4">::</span><span class="me2">Y</span> y<span class="br0">)</span>
    <span class="br0">{</span>
        h<span class="br0">(</span>y<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls B::h (endless recursion): ADL examines the A namespace</span>
              <span class="co1">// but finds no A::h, so only B::h from ordinary lookup is used</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/33.html">CWG 33</a>
</td>
<td> C++98
</td>
<td> the associated namespaces or classes are unspecified if an argument used for<br>lookup is the address of a group of overloaded functions or a function template
</td>
<td> specified
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/90.html">CWG 90</a>
</td>
<td> C++98
</td>
<td> the associated classes of a nested non-union class did not include its<br>enclosing class, but a nested union was associated with its enclosing class
</td>
<td> non-unions also associated
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/239.html">CWG 239</a>
</td>
<td> C++98
</td>
<td> a block-scope function declaration found in the ordinary<br>unqualified lookup did not prevent ADL from happening
</td>
<td> ADL not considered except<br>for using-declarations
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/997.html">CWG 997</a>
</td>
<td> C++98
</td>
<td> dependent parameter types and return types were excluded from consideration<br>in determining the associated classes and namespaces of a function template
</td>
<td> included
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1690.html">CWG 1690</a>
</td>
<td> C++98<br>C++11
</td>
<td> ADL could not find lambdas (C++11) or objects<br>of local class types (C++98) that are returned
</td>
<td> they can be found
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1691.html">CWG 1691</a>
</td>
<td> C++11
</td>
<td> ADL had surprising behaviors for opaque enumeration declarations
</td>
<td> fixed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1692.html">CWG 1692</a>
</td>
<td> C++98
</td>
<td> doubly-nested classes did not have associated namespaces<br>(their enclosing classes are not members of any namespace)
</td>
<td> associated namespaces are<br>extended to the innermost<br>enclosing namespaces
</td></tr></table>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="lookup.html" title="cpp/language/lookup">Name lookup</a> 
</li><li> <a href="function_template.html" title="cpp/language/function template">Template argument deduction</a>
</li><li> <a href="overload_resolution.html" title="cpp/language/overload resolution">Overload resolution</a>
</li></ul>
<h3><span class="mw-headline" id="External_links">External links</span></h3>
<ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><a href="adl.html#cite_ref-1">↑</a></span> <span class="reference-text">Andrew Koenig: <a rel="nofollow" class="external text" href="https://www.drdobbs.com/cpp/a-personal-note-about-argument-dependent/232901443">"A Personal Note About Argument-Dependent Lookup"</a></span>
</li>
<li id="cite_note-2"><span class="mw-cite-backlink"><a href="adl.html#cite_ref-2">↑</a></span> <span class="reference-text"> H. Sutter (1998) <a rel="nofollow" class="external text" href="http://www.gotw.ca/publications/mill02.htm">"What's In a Class? - The Interface Principle"</a> in C++ Report, 10(3)</span>
</li>
</ol>

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